There are three general types of stepping motors: the variable reluctance-type, the hybrid-type, and the permanent magnet-type. With an appropriate electronic motor driver (i.e., controller), all three types offer the capability of a wide range of angular stepping or indexing movements and characteristics. A general reference on the control of stepping motors can be found on-line at http://www.cs.uiowa.edu/˜jones/step/ by Douglas W. Jones of the University of Iowa.
The variable reluctance-type (“VR-type”) motors are traditionally built with salient rotor poles and salient stator fingers (or teeth), but without magnets. A VR-type motor is known for its lack of detent torque and low torque density. In order to improve torque density, both hybrid-type and permanent magnet-type stepping motors use permanent magnet(s) on the moving member (e.g., rotor) and/or stationary member (e.g., stator). They are indistinguishable from the motor driver's point of view. Traditionally, the rotor of a hybrid-type stepping motor is built with a donut-shaped magnet at the center of two rotor disks, which results in substantially axial flux flow from the magnet to the two rotor disks.
The stepping intervals of hybrid-type stepping motors are, typically, about 0.9° per step (i.e., for a motor having 100 rotor poles per rotor disk), or about 1.8° per step (i.e., for a motor having 50 rotor poles per rotor disk), or even larger. The inherent mechanical resonances associated with the step movement increases with step interval and rotor inertia. For smaller step intervals, the rotor sizes have to be increased to accommodate the increased number of required rotor poles. This leads to increased motor size, weight and cost. The use and handling of a large donut magnet for the rotor of the hybrid-type can be problematic due to the strong magnetic force and the fragile nature of magnets. A conventional solution for smaller step intervals and smoother step movement is to use a microstep motor controller to reduce the step interval from the full cardinal step to ½, ¼, ⅛, 1/16, or even smaller fractions, of a full cardinal step. However, microstepping is known for unequal step intervals and erratic jerks in rotor motions. A thorough review of microstepping can be found in US 2007/0013237 A1.
Various stepping motor designs involving permanent magnets are derived from magnetic circuit manipulation of poles and magnets. Among the motor designs that are relevant to this improved motor, Mastromattei (U.S. Pat. No. 4,713,570), Horber (U.S. Pat. No. 4,712,028), and Gamble (U.S. Pat. No. 4,728,830), have provided magnetically-enhanced variable-reluctance motor designs with permanent magnets sandwiched in stator fingers. Shibayama et al. (U.S. Pat. No. 6,262,508) use magnets in both the stator and rotor to increase the motor torque. Horst (U.S. Pat. No. 6,777,842) uses magnets inside the stator arms to minimize magnet material and manufacturing costs.
However, the permanent magnets in all the referenced prior art designs are associated with the stator finger(s) on the stator arms, and each stator finger is associated with a rotor pole. Due to limitations in the design of small magnets, it is impractical to use the motor designs typified by the foregoing prior patents in designing motors having very small step intervals that typically require a large number of rotor poles and stator fingers.
The stepping motors invented by Schaeffer (U.S. Pats. No. 4,190,779 and 4,315,171), and by Applicant (see, e.g., International Application PCT/US08/010,246), have a large number of alternately-magnetized magnets on the rotor to provide for small step intervals. The stepping intervals are, typically, 1.0° per step, 1.5° per step, or higher, for two-, three- and four-phase motors. These motors offer the advantage of high unpowered and powered detent torques, relatively-short axial motor lengths for small size and weight, small rotor inertias, and large through-hole solutions on the rotor. These motors have found great success in the last thirty years in space applications, such as in powering solar array drives, antenna pointing mechanisms, and other guidance, deployment and positioning systems.
When small step intervals and low motion-related disturbances are desired, the required number of rotor poles dramatically increases. For example, a three-phase, 1.5° per step, bipolar stepping motor may have an 80-pole rotor. However, a three-phase, 1.0° per step, bipolar motor may require a 120-pole rotor. Advanced applications in space, semiconductors, printing devices, and other automation fields may require state-of-the-art stepping motor designs with even smaller step intervals, lower rotor inertia, smoother step movement, higher resolutions, and greater step stability in reduced mechanism size and mass, etc. Further reduction of step interval would require very thin magnets that are easy to break, therefore, adding manufacturing cost and difficulties.